Immunoglobulin variable region genes are assembled from component segments in a highly ordered and regulated process. We are interested in defining the molecular mechanisms which govern the expression and rearrangement of human Ig genes during B cell differentiation. In general, most VH segments are not expressed until VH to DJH rearrangement brings associated promoter elements into proximity of the IgH enhancer (Emu). We have recently described a novel 1.2kb human VH5 gene transcript present in normal fetal liver and in transformed B lineage cells. These transcripts, similar in structure to those described in the mouse, represent unique transcription of VH genes in unrearranged (germline) configuration, outside the influence of Emu. Transcription and translation of murine DJH rearrangements has also been shown to occur (termed Dmu transcripts and proteins). The function of these truncated heavy chain transcripts in unknown. In this application, we propose to investigate the role of these transcripts and encoded proteins in regulating human B cell development. Specifically, we will further characterize human germline VH5 transcripts and determine whether they encode protein. Antibodies will be made for structural and functional studies of germline VH protein. Sequences controlling the transcription of germline VH5 genes will be identified by expression vector transfections into pre-B and other lymphoid and non-lymphoid cell types. These studies will identify the molecular mechanisms which control the lineage and stage-specific expression of germline VH5 genes and investigate the role of potential regulatory elements in controlling Ig rearrangement. We will also characterize human Dmu transcripts, determine which human D segments are able to encode proteins, and analyze their potential effects on the development of the B cell repertoire. Finally, we propose to examine the function of a membrane Dmu complex in pre-B cell development. We postulate that expression of truncated heavy chain transcripts and proteins has important functions in the regulation of normal pre-B cell growth and differentiation.